Tracing formation and durability of calcite in a Punic–Roman cistern mortar (Pantelleria Island, Italy)

Abstract

Ancient hydraulic lime mortar preserves chemical and isotopic signatures that provide important information about historical processing and its durability. The distribution and isotopic composition of calcite in a mortar of a well-preserved Punic–Roman cistern at Pantelleria Island (Italy) was used to trace the formation conditions, durability, and individual processing periods of the cistern mortar. The analyses of stable carbon and oxygen isotopes of calcite revealed four individual horizons, D, E, B-1 and B-2, of mortar from the top to the bottom of the cistern floor. Volcanic and ceramic aggregates were used for the production of the mortar of horizons E/D and B-1/B-2, respectively. All horizons comprise hydraulic lime mortar characterized by a mean cementation index of 1.5 ± 1, and a constant binder to aggregate ratio of 0.31 ± 0.01. This suggests standardized and highly effective processing of the cistern.

The high durability of calcite formed during carbonation of slaked lime within the matrix of the ancient mortar, and thus the excellent resistance of the hydraulic lime mortar against water, was documented by (i) a distinct positive correlation of δ¹⁸O_calcite and δ¹³C_calcite; typical for carbonation through a mortar horizon, (ii) a characteristic evolution of δ¹⁸O_calcite and δ¹³C_calcite through each of the four mortar horizons; lighter follow heavier isotopic values from upper to lower part of the cistern floor, and (iii) δ¹⁸O_calcite varying from −10 to −5 ‰ Vienna Pee Dee belemnite (VPDB). The range of δ¹⁸O_calcite values rule out recrystallization and/or neoformation of calcite through chemical attack of water stored in cistern.

The combined studies of the chemical composition of the binder and the isotopic composition of the calcite in an ancient mortar provide powerful tools for elucidating the ancient techniques and processing periods. This approach helps to evaluate the durability of primary calcite and demonstrates the importance of calcite as a proxy for chemical attack and quality of the ancient inorganic binder.

Keywords:

• calcite
• carbon-13
• hydraulic lime mortar
• hydrogen-2
• isotope fractionation: isotope geochemistry
• oxygen-18
• Pantelleria
• Punic–Roman cistern

Acknowledgements

The EMPA analyses were carried out utilizing Jeol JXA8200 equipment at the Montan University in Leoben (Austria) within UZAG cooperation. Chemical analyses were conducted at NAWI Graz Central Lab for Water, Minerals and Rocks. The constructive comments of two anonymous reviewers are greatly appreciated.

Disclosure statement

No potential conflict of interest was reported by the author(s).